Cache Coherence Introduction Quiz

Questions and Answers

What is the main goal of maintaining coherence among caches in a multi-core system?

To ensure all cores agree on the order of writes

What is the main drawback of having no caches in a multi-core system?

Poor system performance

What is the main issue with private write-through caches in maintaining coherence?

Caches may not see writes from other cores

What is the main difference between write-update and write-invalidate coherence?

Write-update broadcasts writes to all caches, while write-invalidate invalidates all other caches Signup and view all the answers

What is the purpose of an 'ordering point' in directory-based coherence?

To determine the order of writes for a block Signup and view all the answers

What is the main potential drawback of snooping-based coherence?

The shared bus may become a bottleneck Signup and view all the answers

What is the main purpose of cache coherence?

To ensure that shared memory behaves like one single memory Signup and view all the answers

Why are private (per-core) L1 caches used in a system instead of one large L1 cache shared among all cores?

To prevent bottlenecking due to multiple core accesses Signup and view all the answers

What issue arises when a core updates its cache but doesn't write the updated value back to main memory?

Other cores still read outdated data from their caches Signup and view all the answers

What happens in a scenario where one core writes a value to its cache, but another core always reads a stale value from its own cache?

The system fails to achieve shared memory behavior Signup and view all the answers

How does the requirement for cache coherence help avoid inconsistent shared memory behavior?

By ensuring that only one core has write access at a time Signup and view all the answers

What condition must be met regarding core interactions for a read operation to return the most recent write value according to cache coherence requirements?

No other core can perform a write operation in parallel Signup and view all the answers

Study Notes

Cache Coherence

Cache coherence is necessary to ensure that when one core writes to its cache, other cores can see the updated value when they read from their own caches.

Cache Coherence Problem

The problem arises when each core has its own private L1 cache, and a write to one cache is not immediately reflected in other caches.
This leads to incoherent shared memory, where the same memory location has different values when seen from different cores.

Requirements for Cache Coherence

Three requirements for cache coherence:
- Reads from the same core see the most recent writes from the same core.
- If one core writes to a location and another core reads from the same location after a sufficient time, the read returns the value from the write.
- If multiple writes are made to the same location, these writes must be serialized, and all cores must see the same order of writes.

Approaches to Maintaining Cache Coherence

Two approaches to maintain coherence property 2:
- Write-update coherence: broadcast writes to update other caches.
- Write-invalidate: writes prevent hits to other copies.
Two approaches to maintain coherence property 3:
- Snooping: writes are broadcast on a shared bus, and the order is seen by all cores.
- Directory: each block has an "ordering point" that figures out who to update and invalidate.